PROJECT SUMMARY While the advent of antiretroviral therapy (ART) has dramatically reduced the morbidity and mortality associated with HIV infection, viral eradication is not achievable due to the persistence of latently-infected cells during treatment. Accumulating data suggest that HIV-infected individuals often experience persistent immune dysregulation, chronic inflammation, and accelerated aging even in the setting of ART-mediated viral suppression. These realities have created a pronounced interest in developing strategies to eradicate HIV in infected individuals. Ample data suggest that the central nervous system (CNS) is a critical anatomical site supporting HIV persistence during ART, likely due to the immune privileged nature of the CNS and the suboptimal penetration of many antiretroviral drugs across the blood-brain barrier. Therefore, it is imperative that emerging HIV eradication strategies are comprehensively examined within the CNS compartment. Our group has recently demonstrated that the human immunomodulatory carbohydrate-binding protein ?galectin-9? (Gal-9) is a determinant of HIV persistence in HIV-infected individuals on suppressive ART. Administration of Gal-9 potently reactivates latent HIV in blood-derived CD4+ T cells ex vivo, by signaling through specific glycans on the cell surface to modulate key host factors that regulate HIV transcription. Furthermore, Gal-9 induces the APOBEC3 host restriction factors which lethally mutagenize the HIV genome, attenuating viral infectivity and minimizing the probability that the HIV reservoir will be replenished when latency is reversed therapeutically. These data suggest that Gal-9 may serve as a foundation for durable virologic control and novel curative approaches, including the ?shock-and-kill? HIV eradication framework. Extensive published data describe a highly important role of human Gal-9 in the biology of the CNS, although relevance to CNS HIV persistence is as yet unknown. In this R01 project, we will evaluate the capacity of exogenous galectin-9 to achieve latent viral reactivation and viral clearance in the CNS compartment, and elucidate the role of endogenous galectin-9 in CNS HIV persistence. In Aim 1, we will evaluate the effects of recombinant galectin-9 (rGal-9) on the establishment and reversal of HIV latency in the CNS using a cutting- edge dual-reporter virus construct and microglial latency model, respectively. In Aim 2, we will determine if rGal-9 administration initiates an antiviral state in the CNS that promotes sustained virologic control, characterizing effects on cell-intrinsic host restriction mechanisms and antiviral cytokine secretion. In Aim 3, we will determine if endogenous Gal-9 expression levels in cerebrospinal fluid and brain tissue are associated with HIV reservoir size in the CNS, leveraging a unique HIV and Sudden Cardiac Death cohort at UCSF. This study will yield valuable insights into how Gal-9 and glycan-dependent signaling at the cell surface may be exploited to achieve viral clearance in the CNS compartment, a critical step in the search for an HIV cure.